LOWER SILURIC SHALES OF THE MOHAWK VALLEY 4! 



The constant alternation of more or less coarse sandstone with 

 shales is indicative of a frequent shifting of the conditions, presum- 

 ably through currents, either reversal (tidal) or continuous currents. 

 There is sometimes clear evidence of absolutely regular or rhythmic 

 shifting. Such a place was for instance observed in an abandoned 

 quarry on the Bozen kill between Altamont and Delanson. The 

 base is here formed by a compact bed of sandstone some 15 feet 

 thick. This sandstone is abruptly followed by dark argillaceous 

 shale in which higher up thin sandstone layers appear, that become 

 more frequent until another thick sandstone bed is formed, like the 

 basal one. This in turn is cut off by a shale, that gradually yields 

 to sand. The whole cycle is in this place repeated three times, 

 shales and sandstones being each of equal thickness, the whole in- 

 dicating a most remarkable regularity of change of deposition which 

 on account of the very shallow water character of the rocks of that 

 locality, may well have been a condition due to reversal or tide 

 currents. 



It agrees well with the evidence of the shallow water or shore 

 origin of the Schenectady shales and sandstones, that they are often 

 densely filled with remains of seaweeds ( Sphenophycus 

 1 a t i f o 1 i u s Hall). 



In contrast to the Frankfort shales the Schenectady beds have 

 furnished a large fauna, although they had hitherto currently been 

 held to be quite barren. Altogether about 70 species have been 

 found and there appears to be no striking difference in the lower 

 and upper faunas of this great mass of rocks, aside from that of 

 the Indian Ladder beds, which have been separated as a distinct 

 formation. 



The following are the faunal lists from the principal localities. 



The easternmost outcrops which represent very low beds of the 

 formation are those at Aqueduct and Rex ford Flats, three miles 

 northeast of Schenectady; these have afforded the following asso- 

 ciation of species: 



Sphenophycus latifolius (Hall) (cc) 



Azygograptus sp. nov. (r) 



Mastigograptus v/. simplex Walcott (fragment) (rr) 



Climacograptus typicalis Hall (c) 



Diplograptus vespertinus Rued. (c) 



Lingula rectilateralis Emmons (rr) 



Leptobolus insignis Hall (c) 



Rafmesquina alternata (Conrad) (small) (c) 



Serpulites (rr) 



